Procedures are being developed for analysis of N15 and C13 in biologically important substances by nuclear magnetic resonance spectroscopy. The methods are nondestructive and applicable to mixtures of compounds. The major objective at this time is the development of equipment of greater detection sensitivity which will permit studies of N15 at its low natural-abundance level. With C13, substantial success has already been achieved in this way, and studies have been made of bile acids, alkaloids, vitamins, substances with marijuana-like activity, anticoagulants, enzymes, and pentacyclic triterpenoids. With N15, spectra at the natural-abundance level have been obtained for many amino acids, peptides, a few enzymes, tRNA samples, nucleosides, nucleotides, and alkaloids, although rather large samples are required. The determination of N15 spectra is rendered especially difficult by variable relaxation times and exceedingly variable nuclear Overhauser enhancements. This equipment features Fourier-transform operation with exceptionally large sample diameters (25 mm) and a 45-kilogauss field.